This invention relates generally to electrochemical power cells, and is more particularly concerned with those cells termed primary electrochemical cells.
In copending application Ser. No. 685,214 filed May 11, 1976 (a continuation of Ser. No. 492,316, filed July 26, 1974, now abandoned) and assigned to the assignee of this application, there are described primary electrochemical cells having an oxidizable active anode material, such as lithium, a carbon cathode, and an electrolytic solution between and in contact with the anode and cathode, the electrolytic solution comprising a covalent inorganic oxyhalide solvent, such as phosphorus oxychloride, monofluorophosphoryl dichloride, thionyl chloride, or sulfuryl chloride, and a solute dissolved therein. It has been found that the carbon cathode material catalyzes the electrochemical decomposition of the solvent during discharge of the cell, thereby enabling the otherwise "dead" weight of the solvent to be utilized as a source of energy.
In copending application of Auborn, et al., Ser. No. 515,557, filed Oct. 24, 1974, and assigned to the assignee of this application, now U.S. Pat. No. 3,923,543, issued Dec. 2, 1975, there are described primary electrochemical cells having an oxidizable active anode material, such as lithium, a cathode including, as the active cathode material, an intercalation compound of carbon and fluorine of the general formula (C.sub.4 F).sub.n, and an electrolytic solution between and in contact with the anode and cathode, the electrolytic solution comprising a covalent inorganic oxyhalide solvent selected from those set forth above, and a solute dissolved therein. It has been found that the above-identified intercalation compound will catalyze the electrochemical decomposition of the solvent resulting, unexpectedly, in a cell having a coulombic cathode utilization efficiency greater than 100% of that theoretically attainable according to reduction of the active cathode material. Once again, the otherwise "dead" weight of the electrolytic solvent can be utilized as a source of energy.
In U.S. Pat. No. 3,885,991, issued to Joel R. Finkel on May 27, 1975, and assigned to the assignee of this application, there is disclosed a primary cell in which a unidirectional current flow device is mounted inside the cell and is electrically connected between one of the electrodes and its associated terminal to prevent current flow in the recharging direction.
In U.S. Pat. No. 3,622,397, issued to Louis Belove on Nov. 23, 1971, there is disclosed a rechargeable nickelcadmium cell in which a thermal switch is mounted external to the cell case and electrically connected between the cell and the recharging source to cut off the recharging current upon the attainment of a certain temperature. Since the switch is outside the cell, it does not sense the temperature inside the cell, especially at the center of the cell but, instead, senses the temperature adjacent to, but outside, the cell, case. Furthermore, since it is connected between the cell and the recharging source, it does not prevent the cell from discharging when the preselected temperature is exceeded, but, rather, merely cuts off the recharging current.
It has been found that the safety of the lithium cell depends largely on the rate of discharge, because of IR losses inside the cell. The internal resistance and the rate of discharge determine the heat generation in the cell. If the rate of heat dissipation from the cell to the environment is smaller than the rate of heat generation inside the cell, the cell temperature will increase steadily, if the discharge is continuously maintained to a point where the cell may explode. At this point, some of the cell components (eg. lithium and free sulfur resulting from reduction of the oxyhalide electrolyte) react instantly with each other and all the energy of the system is released at once. This will generally happen if the cell temperature is permitted to exceed the melting points of sulfur and lithium.